I'd like to know if there is any difference between these two sentences:
1) The train travels from Moscow to St.Petersburg by night
2) The train travels from Moscow to St.Petersburg at night
The intended meaning is that the train leaves at 23.50 and arrives in St.Petersburg at 8.00 AM so that people sleep on the train.
The same question about the following sentences: Both are correct, the first one has, perhaps, more of a suggestion that the journey takes all night.
1) 3 years ago I used to work by day (from morning to evening)
2) 3 years ago I used to work at the daytime (from morning to evening) This one is incorrect, we say "in the daytime".
Please tell me which sentences are correct and which are not.